In recent years it has been found that certain materials can be very finely dispersed in polyamide matrices to render the resultant composition "tough". It has been known for many years that ethylene-carboxylic acid copolymers can be dispersed in polyamides to improve toughness by reaction of the acid component of the copolymer with the amine ends of the polyamide chain. However, these materials are only toughened to a modest extent. More recently low modulus materials with reactive sites described in Epstein U.S. Pat. No. 4,174,358 have a toughness an order of magnitude higher than the "base" polyamide resin; and when measured for toughness, break in a ductile rather than brittle fashion. These extra tough resins are highly susceptible to loss of toughness by addition of any "foreign" bodies such as glass fibers for reinforcement, and fire retardants. In fact, whereas glass fibers increase the toughness of simple polyamides by a factor of two, similar glass fibers reduce the toughness of these extra tough materials by a factor of four. Similar considerations apply to additives which are used to make the resins fire retardant, particularly when sufficient additive is used to meet the very stringent requirements to achieve a "V-O" rating in the Underwriters Laboratory Test 94 for fire retardancy.
Thus, even when satisfactory fire retardant additives for polyamides such as those described in Mohajer U.S. Pat. No. 3,808,171 are used they either fail to pass the flammability test or drastically reduce the toughness of the extra tough material.
The Mohajer patent describes a class of halogenated organic flame retardants that allegedly do not degrade or discolor polyamide resins, and therefore do not weaken them, when added to the polyamide resins in the absence of toughening agents. But when toughening agents, especially adducts of an unsaturated compound containing carboxyl groups and a polymer of ethylene and a nonconjugated diene, are added to polyamide resins, the teaching of Mohajer is not applicable. For example, when bis(p-bromo-phenyl)ether (a flame retardant of Mohajer) is added to a blend of a polyamide and such a toughening adduct, the resulting mixture quite surprisingly is not flame retardant. Applicant, however has found that when octabromo diphenyl ether is added to a blend of a polyamide and the adduct, the resulting mixture is both tough and flame retardant.